Boating Weather: Thermal Sea and Land Breezes

All summer long I’ve been describing the southwesterly sea-breeze conditions typically found on Narragansett Bay to my boating guests. While the shape of the bay and the surrounding land contributes greatly to the result, the primary cause of the sea breeze is the rising of warm air and the sinking of colder, denser air. More scientifically, this kind of sea breeze is called a “thermal” wind and the same term applies to the reverse air-flow at night, or thermal land breeze.

As sunlight warms the land during the day, the air above the land loses density and rises, drawing in cooler, denser air from over the water. Illustration courtesy of WikiMedia Commons.

According to Knight’s Modern Seamanship, water has more capacity to absorb heat than any solid or liquid except ammonia. Water’s unique capacity to store heat — but without a large change in its own temperature — accounts for the reason that land is warmer than its adjacent waters during summer. This affects large inland lakes as well as coastal regions. Air over the land is heated, becomes less dense, and expands upward, pulling cooler air onshore from the surface of the surrounding water. These cooling sea breezes can penetrate inland for considerable distances (25 miles) even though they extend only a few hundred feet above the ground, while the same condition on inland lakes may penetrate for just a few miles.

At night, radiational cooling may cause the land to become cooler than air over the water, so air-flow in the opposite direction — a land breeze — occurs. Without the sun, nighttime differences between land and water temperature are not as dramatic, and hence the land breeze is typically not as strong as a sea breeze.

The thermal conditions in Newport, Rhode Island are so regular that you can almost set your watch by them. The southwest sea breeze comes in around noon and produces fair-weather clouds. Regardless of the overall weather pattern or wind direction, the sea-breeze effect is so dominant that it blows until the land cools around sunset. And all this is because of the uneven heating of land and water, or more specifically because water has a heat capacity of two or three times that of land.